This invention relates to a method of, and apparatus for, assessing characteristics of fluids. More particularly, it relates to the quantitative assessment of a change in a fluid, e.g. dilution or contamination thereof, or chemical change therein, which change alters the characteristics of the fluid with respect to transmission of radiation therethrough.
The invention is particularly applicable to the assessment of the concentration of a substance present in a fluid, the fluid being relatively transparent to radiation and the substance in the fluid causing an increasing resistance to the passage of radiation through the fluid as its concentration increases. The invention has been devised in relation to the situation wherein the fluid is a liquid and the substance therein is another liquid, such two liquids respectively being substantially transparent to visible light and resistant to the passage of visible light therethrough, such that increasing concentration of the latter liquid in the former liquid causes a progressive decrease in the transparency of the mixed liquids to light. However, it will be appreciated that the principle of the invention is also applicable when the substance whose concentration is required to be assessed is a solid which may form a solution or suspension in the liquid to cause a progressive decrease in the transparency of the liquid with increasing concentration of the solid. The principle of the invention is also applicable to mixtures of gases or vapours wherein a change in the concentration of one such gas or vapour in another causes a change in the transparency of the mixture to light or other radiation, or to the presence of particles in gases, e.g. smoke.
Some examples of applications of the invention are set forth hereafter, but the invention has been devised in relation to a requirement which exists in the medical field, namely that of assessing the quantity of blood contained in a liquid which otherwise comprises wholly or mainly water. A typical situation in which the requirement arises is that where surgery is performed on the prostate glad of a patient. It is important to know how much of the patient's blood is lost in order to asses the need for replacement thereof by transfusion, but direct measurement of such blood loss is extremely difficult because, although fluid can be collected by a catheter inserted into the bladder of the patient, such fluid comprises blood mixed with some urine from the patient's bladder and irrigation fluid, which is mainly water, introduced into the patient's bladder. The drained fluid, collected in a catheter bag, can be subjected to laboratory analysis to determine the concentration of blood therein, and thus the requirement for replacement of blood, but such analysis is time consuming and expensive. Furthermore, highly accurate analysis is not necessary for the immediate purpose of assessing the requirement for replacement blood. There is thus a requirement for a rapid and reasonably accurate assessment of the concentration of blood in the drained fluid to be made; then, since the total volume of fluid in a catheter bag is readily measured, it is possible to make a virtually immediate assessment of any requirement for blood replacement